Charge injection noise reduction in sample-and-hold circuit

ABSTRACT

A sample-and-hold circuit includes a first transistor; a second transistor disposed between a gate electrode and a drain electrode of the first transistor; a sampling capacitor, an electrode of the sampling capacitor being connected to the gate electrode of the first transistor; and a first current source connected to the drain electrode of the first transistor, where a gate electrode of the second transistor receives a gate control signal. A minimum voltage of the gate control signal is V th2 +V sat2 +V th1 +V sat1 , where V th1  is a threshold voltage of the first transistor, V sat1  is a saturation voltage of the first transistor, V th2  is a threshold voltage of the second transistor, and V sat2  is a saturation voltage of the second transistor.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a Continuation of application Ser. No.14/928,524, filed Oct. 30, 2015. The entire contents of theseapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This application relates generally to sample-and-hold circuits. Morespecifically, this application relates to a sample-and-hold circuit thatcan reduce the effects of noise in image sensing or other electronicapplications.

2. Description of Related Art

Image sensing devices typically consist of an image sensor, generally anarray of pixel circuits, as well as signal processing circuitry and anyassociated control or timing circuitry. Within the image sensor itself,charge is collected in a photoelectric conversion device of the pixelcircuit as a result of the impingement of light.

One example of a pixel circuit is illustrated in FIG. 1. As shown inFIG. 1, a pixel circuit 100 includes a photoelectric conversion device101 (for example, a photodiode), a floating diffusion FD, a transfertransistor 102, a reset transistor 103, an amplification transistor 104,and a selection transistor 105, and a vertical signal line 106. Asillustrated, vertical signal line 106 is common to a plurality of pixelcircuits within the same column. Alternatively, a vertical signal linemay be shared among multiple columns. Gate electrodes of transfertransistor 102, reset transistor 103, and selection transistor 105receive signals TRG, RST, and SEL, respectively. These signals may, forexample, be provided by the control or timing circuitry.

While FIG. 1 illustrates a pixel circuit having four transistors in aparticular configuration, the current disclosure is not so limited andmay apply to a pixel circuit having fewer or more transistors as well asother elements, such as capacitors, resistors, and the like.Additionally, the current disclosure may be extended to configurationswhere one or more transistors are shared among multiple photoelectricconversion devices.

The accumulated charge is then converted to a digital value. Such aconversion typically requires several circuit components such assample-and-hold (S/H) circuits, analog-to-digital converters (ADC), andtiming and control circuits, with each circuit component serving apurpose in the conversion. For example, the purpose of the S/H circuitmay be to sample the analog signals from different time phases of thephoto diode operation, after which the analog signals may be convertedto digital form by the ADC.

However, typical sample-and-hold circuit implementations may containvarious stray capacitances and stray charge, which may result ininaccuracies in the output of the sample-and-hold circuit. This isundesirable in applications where a high precision of thesample-and-hold is required. Therefore, there exists a need for a methodof signal processing that can reduce the impact of noise as a result ofstray capacitances and stray charges.

BRIEF SUMMARY OF THE INVENTION

Various aspects of the present disclosure relate to a sample-and-holdcircuit and a gate control circuit for applying a gate control signal toa gate electrode within the sample-and-hold circuit.

In one aspect of the present disclosure, a sample-and-hold circuitincludes a first transistor; a second transistor disposed between a gateelectrode and a drain electrode of the first transistor; a samplingcapacitor, an electrode of the sampling capacitor being connected to thegate electrode of the first transistor; and a first current sourceconnected to the drain electrode of the first transistor, wherein a gateelectrode of the second transistor is configured to receive a gatecontrol signal.

In another aspect of the present disclosure, a gate control circuit,comprises a first transistor; a second transistor connected to the firsttransistor in series; a first current source; and an inverter configuredto receive a supply voltage and an inverter control signal and output agate control signal.

In the above aspects of the present disclosure, a minimum voltage of thegate control signal is V_(th2)+V_(sat2)+V_(th1)+V_(sat1), whereinV_(th1) is a threshold voltage of the first transistor, V_(sat1) is asaturation voltage of the first transistor, V_(th2) is a thresholdvoltage of the second transistor, and V_(sat2) is a saturation voltageof the second transistor.

In this manner, the above aspects of the present disclosure provide forimprovements in at least the technical field of signal processing, aswell as the related technical field of imaging.

This disclosure can be embodied in various forms, including businessprocesses, computer-implemented methods, computer program products,computer systems and networks, user interfaces, application programminginterfaces, hardware-implemented methods, signal processing circuits,image sensor circuits, application specific integrated circuits, fieldprogrammable gate arrays, and the like. The foregoing summary isintended solely to give a general idea of various aspects of the presentdisclosure, and does not limit the scope of the disclosure in any way.

DESCRIPTION OF THE DRAWINGS

These and other more detailed and specific features of variousembodiments are more fully disclosed in the following description,reference being had to the accompanying drawings, in which:

FIG. 1 illustrates an exemplary pixel circuit for use with variousaspects of the present disclosure.

FIG. 2 illustrates an exemplary bottom plate sampling S/H circuitaccording to various aspects of the present disclosure.

FIG. 3 illustrates an exemplary signal timing diagram of the exemplaryS/H circuit according to FIG. 2.

FIG. 4 illustrates an exemplary implementation of a sample-and-holdcircuit using transistors according to various aspects of the presentdisclosure.

FIG. 5 illustrates stray capacitances in a transistor switch in animplementation according to FIG. 4.

FIG. 6 illustrates an exemplary relationship among transistor gatevoltages and an output voltage in an implementation according to FIG. 4.

FIG. 7 illustrates an exemplary sample-and-hold circuit with an externalcircuit for providing a gate control signal according to various aspectsof the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous details are set forth, such asflowcharts, data tables, and system configurations. It will be readilyapparent to one skilled in the art that these specific details aremerely exemplary and not intended to limit the scope of thisapplication.

Moreover, while the present disclosure focuses mainly on examples inwhich the S/H circuits are used in image sensors, it will be understoodthat this is merely one example of an implementation. It will further beunderstood that the disclosed S/H circuits can be used in any device inwhich there is a need to sample a signal and/or compare two voltages;for example, an audio signal processing circuit, industrial measurementand control circuit, and so on.

In this manner, the present disclosure provides for improvements in thetechnical field of signal processing, as well as in the relatedtechnical fields of image sensing and image processing.

[Sample-and-Hold Circuit]

FIG. 2 illustrates an exemplary analog S/H circuit 200, of a bottomplate sampling type. This illustrative S/H circuit comprises anamplifier 205, a sampling capacitor 204, and switches 201-203. In thisexample, V_(ref1) is a reference voltage and V_(in) is the input analogvoltage (that is, the input signal) to be sampled. In an image sensorimplementation, V_(in) represents a pixel value. Switches 201-203 arepreferably transistors, such as CMOS transistors.

In operation, switches 201-203 are controlled according to a particulartiming by control signals SW1-SW3. That is, switch 201 is controlled bya control signal SW1, switch 202 is controlled by a control signal SW2,and switch 203 is controlled by a control signal SW3. FIG. 3 illustratesan exemplary timing diagram for the operation of S/H circuit 200, andshows the respective waveforms of control signals SW1-SW3. In FIG. 3,for purposes of illustration, “high” signals indicate “closed” (i.e.,connected) switches and “low” signals indicate “open” (i.e.,disconnected) switches.

At the beginning of the illustrated period, the signal V_(in) issampled. During this period, signals SW1 and SW2 are high and signal SW3is low. Thus, switches 201 and 202 are closed, whereas switch 203 isopen. This causes capacitor 204 to be charged to the voltageV_(in)(t1)-V_(ref1), where t1 is the time where the capacitor ischarged. After capacitor 204 has been charged, signal SW1 becomes low,while signal SW2 remains high and signal SW3 remains low. Thus, switch201 is opened, while switch 202 remains closed and switch 203 remainsopen. This disconnects the feedback path of amplifier 205. The voltageat capacitor 204 remains at the level of the previous stage, i.e.V_(in)(t1)−V_(ref1). Then, signals SW2 and SW3 are reversed. That is,while switch 201 remains open, switch 202 becomes open and switch 203becomes closed. The voltage V_(c) on capacitor 204 and the feedbackconnection on amplifier 205 cause the output voltage V_(out) ofamplifier 205 to be the same as V_(in). That is,V_(out)=V_(c)+V_(ref1)=V_(in)(t1)−V_(ref1)+V_(ref1)=V_(in)(t1).

FIG. 4 illustrates an implementation of S/H circuit 200 usingtransistors. In FIG. 4, S/H circuit 400 includes an amplifier andswitches, each of which are implemented using NMOS transistors. Thus,S/H circuit 400 includes NMOS transistors 401-404, a sampling capacitor405, and a current source 406. Transistors 402-404 are controlled viatheir gate voltages. Using transistor 402 as an example, when the gatevoltage V_(g2) slightly exceeds the transistor threshold voltageV_(th2), transistor 402 conducts current and behaves like a resistor. Inother words, transistor 402 operates in the “linear” or “ohmic” region.Similarly, transistors 403 and 404 are turned on and off by applyingsuitable gate voltages V_(g3) and V_(g4), respectively.

The timing and operation of S/H circuit 400 follows a similar switchingsequence as described earlier with regard to S/H circuit 200. That is,signal SW1 is applied to the gate electrode of transistor 402, signalSW2 is applied to the gate electrode of transistor 403, and signal SW3is applied to the gate electrode of transistor 404.

At the beginning of the sampling period, transistors 402 and 403 areturned on, whereas transistor 404 is turned off. In this step, samplingcapacitor 405 is charged. Because transistor 402 is in the on state(i.e., conducting current), the gate bias voltage causes a smallquantity of charge to reside in the channel of transistor 402.Subsequently, when transistor 402 is turned off, the charge intransistor 402 flows to the output node V_(out) and sampling capacitor405 via the stray capacitances 501 (between the gate and the source oftransistor 402) and 502 (between the gate and the drain of transistor402), as illustrated in FIG. 5. This is referred to as the “straycharge.” The stray charge flowing to capacitor 405 changes its voltageby a small amount. That is, the gate voltage of transistor 401 becomesV_(g1)+Δ where Δ is a small voltage variation that is dependent on anumber of factors. A is represented as a function of these factorsf(V_(th1), V_(H)-V_(L), slope(V_(g2))). In the above relations, V_(g1)is the gate voltage if there exist no stray capacitances or straycharges, V_(th1) is the threshold voltage for output transistor 401 tobegin conducting, V_(H) is the high voltage at the gate of transistor402, V_(L) is the low voltage at the gate of transistor 402,slope(V_(g2)) is the rate of change of the gate voltage in transistor402, and f indicates that there is some form of functional dependence onthe arguments.

Even though A may be small, it still causes inaccuracies in the outputof S/H circuit 400. This is undesirable in applications where a highprecision of the S/H circuit is required. Moreover, in S/H circuitimplementations using a feed forward method for KTC noise cancellation,even a small variation in V_(g1) may cause a large change in the outputand a result clipping may occur in the output.

FIG. 6 illustrates the relationship among gate voltages V_(g1) andV_(g2) of transistors 401 and 402, respectively, and the output voltageV_(out) at the drain electrode of transistor 401 for S/H circuit 400 asillustrated in FIG. 4. Specifically, FIG. 6 illustrates thecorresponding changes in V_(g1) and V_(out) as V_(g2) is graduallyincreased from 0. Thus, for illustration purposes, the horizontal axisof FIG. 6 may be viewed as a time axis in the following non-limitingexample, provided for purposes of illustration.

The initial conditions of this example are such that V_(g2) oftransistor 402 in S/H circuit 400 is 0 V. In this case, both transistors401 and 402 are in a non-conducting state, and as a result V_(out) isequivalent to the power supply voltage V_(dd). As V_(g2) is graduallyincreased as illustrated in FIG. 6, it will at some point exceedthreshold voltage V. At this point, transistor 402 beings conductingcurrent. Because of the circuit arrangement of S/H circuit 400, gatevoltage V_(g1) of transistor 401 begins to follow V_(g2), but stays at alevel V_(th2) below V_(g1) V_(g2)=V_(g1)+V_(th2)). During this period,transistor 401 is still not conducting because V_(g1) is below V_(th1).Once V_(g2) exceeds V_(th1) V_(th2), the gate voltage V_(g1) rises aboveV_(th1) and transistor 401 also begins to conduct current. As a result,V_(out) drops from V_(dd). The relationship V_(g2)=V_(g1)+V_(th2) holdsuntil transistor 401 enters the saturation region, at which point V_(g1)starts to flatten out as illustrated in FIG. 6. As V_(g2) continues toincrease, at some point V_(g1) will reach V_(th1)+V_(sat1), whereV_(sat1) is the saturation voltage of transistor 401. At this point,transistor 401 is operating in the saturation region and V_(out) ismaintained at a level equal to V_(th1) V_(sat1).

For S/H circuit 400, the minimum gate voltage V_(g2min) of transistor402 that is required for transistor 401 to conduct in the saturationregion defined by the relationship V_(g2min)=V_(sat1)+V_(th2). Forpurposes of controlling S/H circuit 400 so that it functions properly,it is possible to swing the gate voltage V_(g2) of transistor 402between 0 and a pre-selected “high” level larger than this minimumvoltage V_(g2min). However, if the high level for V_(g2) issignificantly larger than V_(g2min), the charge injection noise mayincrease because a higher voltage at the gate of a transistor generallyresults in more charges being accumulated in the conduction channelthereof. Therefore, it is preferable to swing voltage V_(g2) between 0and a level only slightly higher than V_(g2min) for the purposes ofturning on and off transistor 402. It is preferable to use a levelhigher than V_(g2min), rather than V_(g2min) itself, to account forcomponent variations, circuit noise, and the like. Thus, the high levelis preferably V_(g2min) V_(small). Preferred values of V_(small) will bediscussed in more detail below.

While S/H circuit 400 is described using an example where transistors401-404 are NMOS transistors, S/H circuit 400 can alternatively beimplemented using PMOS transistors. In other words, transistors 401-404may all be PMOS transistors. In such a case, the polarities of thevoltages in the circuit will be reversed, and the direction of thecurrent source 406 will also be reversed to maintain the operation ofS/H circuit 400. Similarly, in such a case the polarities of thevoltages in the graphs of FIG. 6 will also be reversed.

In order to properly control the operation of S/H circuit 400, then, agate control circuit 700 is preferably used as illustrated in FIG. 7.For purposes of clarity in the illustration of FIG. 7, transistors 403and 404 are omitted from this particular illustration. Gate controlcircuit 700 is utilized so that the necessary voltage for V_(g2) may beprovided, thus ensuring that S/H circuit 400 functions correctly and atthe same time has a low injection noise level. Gate control circuit 700includes transistors 701 and 702 connected in series, an inverter 703,and a current source 706. Inverter 703 uses a supply voltage V_(s), andthe output of inverter 703 is determined by the input control signalCTL, which is a 2-level (i.e., 1-bit) control signal. The exact high andlow levels of CTL are subject only to the requirements that the highlevel is sufficiently high to cause the output of inverter 703 to be alow state, and the low level is sufficiently low to cause the output ofinverter 703 to be a high state (i.e., supply voltage V_(s)). The outputof inverter 703 is supplied as V_(g2) to the gate of transistor 402.

Transistors 701 and 702 and current source 706 are matched to thecircuit components of S/H circuit 400. That is, transistors 401 and 701are matched so that they have the same characteristics; transistors 402and 702 are matched so that they have the same characteristics, andcurrent sources 406 and 706 are matched so that they supply the samecurrent I. Since transistors 401 and 402 are NMOS transistors, itimplies that transistors 701 and 702 should also be NMOS transistors.Transistors 401 and 402 are provided in a shorted state with therespective drains and gates thereof shorted. This ensures that bothtransistors 401 and 402 are in the on state. In this manner, V_(s) isgiven by the relation V_(s)=V_(th2a) V_(sat1a) V_(th1a) V_(sat1a), whereV_(th1a) and V_(th2a) are the threshold voltages of transistors 701 and702, respectively, and V_(sat1a) and V_(sat1a) are the saturationvoltages of transistors 701 and 702, respectively. Because thetransistors in gate control circuit 700 are matched to theircounterparts in S/H circuit 400, the above relation is equivalent toV_(s)=V_(th2) V_(sat2) V_(th1) V_(sat1)=V_(g2min) V_(sat2).

When V_(s) is used in this manner via inverter 703 to control transistor402, the switching of transistor 402 is accomplished by applying a highor low level signal as CTL. Thus, from gate control circuit 700, thedesired level V_(g2min) plus a small voltage (V_(small) as above) isapplied to the gate of transistor 402. In the preferred embodimentdescribed above, this small voltage equals the saturation voltageV_(sat2) of transistor 402. Thus, by using gate control circuit 700 withmatched circuit components, the appropriate voltage is provided toswitch transistor 402 on and off so that S/H circuit 400 functionscorrectly and with a lower charge injection noise.

As noted above, the S/H circuit 400 of FIG. 4 may alternatively beimplemented using PMOS transistors. In this case, gate control circuit700 of FIG. 7 can likewise be implemented using PMOS transistors. Insuch an embodiment, all the transistors are PMOS transistors, includingtransistors 701 and 702. Furthermore, the polarities of the voltagesshould be reversed, the direction of the current sources 406 and 706should be reversed, and the polarity of the CTL signal should bereversed.

CONCLUSION

With regard to the processes, systems, methods, heuristics, etc.described herein, it should be understood that, although the steps ofsuch processes, etc. have been described as occurring according to acertain ordered sequence, such processes could be practiced with thedescribed steps performed in an order other than the order describedherein. It further should be understood that certain steps could beperformed simultaneously, that other steps could be added, or thatcertain steps described herein could be omitted. In other words, thedescriptions of processes herein are provided for the purpose ofillustrating certain embodiments, and should in no way be construed soas to limit the claims.

Accordingly, it is to be understood that the above description isintended to be illustrative and not restrictive. Many embodiments andapplications other than the examples provided would be apparent uponreading the above description. The scope should be determined, not withreference to the above description, but should instead be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled. It is anticipated andintended that future developments will occur in the technologiesdiscussed herein, and that the disclosed systems and methods will beincorporated into such future embodiments. In sum, it should beunderstood that the application is capable of modification andvariation.

All terms used in the claims are intended to be given their broadestreasonable constructions and their ordinary meanings as understood bythose knowledgeable in the technologies described herein unless anexplicit indication to the contrary in made herein. In particular, useof the singular articles such as “a,” “the,” “said,” etc. should be readto recite one or more of the indicated elements unless a claim recitesan explicit limitation to the contrary.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed embodiment. Thus the following claims arehereby incorporated into the Detailed Description, with each claimstanding on its own as a separately claimed subject matter.

What is claimed is:
 1. An image sensor, comprising: a pixel circuitconfigured to generate a pixel signal; a vertical signal line configuredto convey the pixel signal; a sample-and-hold circuit configured toreceive the pixel signal, the sample-and-hold circuit, including: afirst transistor, a second transistor disposed between a gate and adrain of the first transistor, a sampling capacitor, an electrode of thesampling capacitor being connected to the gate of the first transistor,and a first current source connected to the drain of the firsttransistor; and a gate control circuit configured to provide a gatecontrol signal to a gate of the second transistor, wherein the gatecontrol circuit includes an inverter configured to receive a supplyvoltage and an inverter control signal, and output the gate controlsignal.
 2. The image sensor according to claim 1, wherein a minimumvoltage of the gate control signal is V_(th2)+V_(sat2)+V_(th1)+V_(sat1),wherein V_(th1) is a threshold voltage of the first transistor, V_(sat1)is a saturation voltage of the first transistor, V_(th2) is a thresholdvoltage of the second transistor, and V_(sat2) is a saturation voltageof the second transistor.
 3. The image sensor according to claim 1,wherein the gate control circuit comprises a third transistor, a fourthtransistor connected to the third transistor in series, and a secondcurrent source.
 4. The image sensor according to claim 3, wherein thethird transistor, the fourth transistor, and the second current sourceprovide the supply voltage to the inverter.
 5. The image sensoraccording to claim 3, wherein the first transistor and the thirdtransistor are matched so as to have the same transistor characteristicsas one another, the second transistor and the fourth transistor arematched so as to have the same transistor characteristics as oneanother, and the first current source and the second current source areconfigured to output the same current as one another.
 6. The imagesensor according to claim 3, wherein a gate and a drain of the thirdtransistor are connected to one another, and a gate and a drain of thefourth transistor are connected to one another.
 7. The image sensoraccording to claim 3, wherein the third transistor and the fourthtransistor are NMOS transistors.
 8. The image sensor according to claim3, wherein the third transistor and the fourth transistor are PMOStransistors.
 9. The image sensor according to claim 1, wherein in a casewhere the inverter control signal is at a higher level of two voltagelevels, the inverter is configured to output a first state voltage, andin a case where the inverter control signal is at a lower level of thetwo voltage levels, the inverter is configured to output a second statevoltage.
 10. The image sensor according to claim 9, wherein the secondstate voltage is the supply voltage, and the first state voltage islower than the supply voltage.
 11. The image sensor according to claim1, wherein the first transistor and the second transistor are NMOStransistors.
 12. The image sensor according to claim 1, wherein thefirst transistor and the second transistor are PMOS transistors.
 13. Theimage sensor according to claim 1, wherein the pixel circuit comprises aphotoelectric conversion device, a floating diffusion, a transfertransistor, a reset transistor, and an amplification transistor.
 14. Theimage sensor according to claim 13, wherein the pixel circuit comprisesa selection transistor.